Traditionally, handguns have included a metal frame and receiver as a single component to which additional components are attached, such as the fire control group, barrel, slide, safety levers, grips, and other parts of the handgun. The advent of polymer-framed handguns has been accompanied by new challenges in firearms design. Instead of a steel frame that also serves as the receiver, polymer-framed handguns commonly have a polymer frame that extends along the barrel and includes a hand grip and trigger guard. A separate metal receiver is installed into a receiver box defined in the top of the frame. The receiver includes components of the fire control group. A slide, often made of metal, attaches to and moves along rails on top of the receiver. Many polymer-framed handguns are striker-fired, where the action includes a striker held in spring tension until released forward to impact the ammunition primer when the user pulls the trigger.